Typically, the writing of optical structures such as Bragg gratings into a photosensitive material involves an interferometer in which two coherent light beams (typically in the UV wavelength range) are directed along separate optical paths and brought to interference substantially within the photosensitive material. Within the photosensitive material, refractive index changes are induced through the interaction between the light beams and the photosensitive material, with the refractive index profile being determined by the interference patterns.
In order to write complex optical gratings, e.g. in an optical fibre, a high contrast, phase stable and arbitrarily chirpable UV interference pattern is typically required. The design problem associated with the creation of such a UV interference pattern for the writing of optical gratings is multi-dimensional and it has been found to be non-trivial to design an optical system that can satisfy all criteria simultaneously while minimising errors in each dimension due to the non-ideal nature of optical and mechanical components.
The present invention seeks to provide an alternative system and method for writing an optical structure in a waveguide, which, in at least preferred embodiments, satisfies the requirements for writing structures with the required flexibility whilst minimising the sum of the errors terms.